The present invention relates to improvements in a driving apparatus for a DC circuit breaker of reverse current insertion type and more particularly to such driving apparatus including a high-speed tripping coil and a relatively low-speed tripping coil which are disposed independently of each other thereby providing a mechanical trip-free mechanism.
Basically, a conventional DC current-limiting circuit breaker of reverse current insertion type disclosed, for example in JP-A No. 54-149873 is constructed as shown in FIG. 1. In other words, this circuit breaker includes a breaker unit 2 inserted in series in a main DC line 1, and a series combination of a commutation capacitor 3, a reactor 4 and a thyristor switch 5 for performing switching operation which is connected in parallel with the breaker unit 2. When performing a circuit-breaking operation, an open command is applied to the breaker unit 2, and after the opening of the breaker unit 2, the thyristor switch 5 is turned on so that the discharge current .circle.2 from the commutation capacitor 3 charged preliminarily from a charge unit 6 is supplied in the reverse direction to the energization current .circle.1 of the breaker unit 2 and thus the sum of the currents .circle.1 and .circle.2 flowing through the breaker unit 2 develops a point of zero. As a result, the arc of the breaker unit 2 is extinguished and the main line current commutates to the commutation capacitor 3, thereby completing the current-limiting process.
While it is necessary to open the breaker unit 2 at a high speed and to cause the commutation capacitor 3 to discharge a high-frequency current in order to break any short-circuit current by the current-limiting action, it has been well known that a vacuum circuit breaker ensuring a satisfactory dielectric strength and an excellent high-frequency current breaking performance with a small opening stroke is suitable for the breaker unit 2, and also it has been well known that a magnetic repulsive coil is suitable for use as the device for opening the breaker unit at a high speed in a short period of time.
By way of example, a circuit breaker comprising a vacuum circuit breaker and employing a magnetic repulsive coil as a high-speed tripping device is constructed as shown in FIG. 2 as disclosed in Japanese Patent Publication No. 56-19942. In other words, it is so constructed that where it is desired to effect the tripping, the energization of a magnetic repulsive coil 7 causes a magnetic repulsive force between it and a short-circuit plate 8 arranged opposite thereto so that a rod 9 connected to the short-circuit plate 8 is moved in the downward direction indicated by an arrow and a movable-side electrode 10 of the vacuum circuit breaker is separated from a fixed-side electrode 11 to move in the opening direction until the movable-side electrode 10 reaches the most-remote maximum stroke position thereby holding the movable-side electrode 10 in the open position by a hook 13 against a closing spring 12 tending to force the movable-side electrode 10 back in the closing direction. In other words, FIG. 2 shows the closed condition of the switch.
This conventional technique is disadvantageous in that basically the construction of a mechanical trip-free mechanism for effecting a breaking operation in preference to the closing operation is not possible due to the open condition being mechanically held by the hook 13, that a greater opening stroke than is necessary is required for ensuring a satisfactory breaking performance and insulating performance due to the engagement of the hook 13, that the input energy to the magnetic repulsive coil 7 is increased with the resulting increase in the burden on the bellows of the vacuum valve, that the high-speed driving involving the operation of the magnetic repulsive coil 7 is performed for every opening operation with the resulting increased stress and increased impact force thereby decreasing the life of the mechanical system on the whole and so on.